This invention relates to separation methods which employ solid inorganic porous materials. More particularly, the present invention relates to novel synthetic ultra-large pore crystalline material useful for separating component substances within a mixture as well as methods relating thereto.
Porous inorganic solids have great utility as catalysts and separation media for industrial applications. Catalytic and sorptive activity are enhanced by the extensive surface area provided by a readily accessible microstructure characteristic of these solids.
The porous materials in use today can be sorted into three broad categories using the details of their microstructure as a basis for classification. These categories are 1) amorphous and paracrystalline supports, 2) crystalline molecular sieves and 3) modified layered materials.
Variations in the microstructures of these materials manifest themselves as important differences in the catalytic and sorptive behavior of the materials, as well as differences in various observable properties used to characterize them. For example, surface area, pore size and variability in pore sizes, the presence or absence of X-ray diffraction patterns, as well as the details in such patterns, and the appearance of the materials when their microstructure is studied by transmission electron microscopy and electron diffraction methods can be used to characterize porous inorganic solids.
Amorphous and paracrystalline materials represent an important class of porous inorganic solids which have been used for many years in industrial applications. Typical examples of these materials are the amorphous silicas commonly used in catalyst formulations and the paracrystalline transitional aluminas used as solid acid catalysts and petroleum reforming catalyst supports.
The amorphous materials are generally characterized as "amorphous" since they are substances having no long range order. Unfortunately, this can be somewhat misleading since almost all materials are ordered to some degree, at least on the local scale. An alternate term which has been used to described these materials is "X-ray indifferent". The microstructure of the silicas consists of 100-250 .ANG. particles of dense amorphous silica (Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, Vol. 20, John Wiley & Sons, New York, p. 766-781, 1982), with the porosity resulting from voids between the particles. Since there is no long range order in these materials, the pore sizes tend to be distributed over a rather large range. This lack of order also manifests itself in the X-ray diffraction pattern, which is usually featureless.
Paracrystalline materials such as the transitional aluminas also have a wide distribution of pore sizes, but exhibit better defined X-ray diffraction patterns usually consisting of a few broad peaks. The microstructure of these materials consists of tiny crystalline regions of condensed alumina phases and the porosity of the materials results from irregular voids between these regions (K. Wefers and Chanakya Misra, "Oxides and Hydroxides of Aluminum", Technical Paper No. 19 Revised, Alcoa Research Laboratories, p. 54-59, 1987).
Despite any differences arising between these paracrystalline or amorphous materials, neither substance has long range order controlling the sizes of pores in the material. Consequently, variability in pore size is typically quite high. The sizes of pores in these materials fall into what is known in the art as the "mesoporous range", which, for the purposes of this Application, is from about 13 to 200 .ANG..
In sharp contrast to these structurally ill-defined solids are materials whose pore size distribution is narrow because it is controlled by the precisely repeating crystalline nature of the materials' microstructure. These materials are referred to as "molecular sieves", the most important examples of which are zeolites.
Zeolites, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction. These crystalline structures contain a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores provide access to molecules of certain dimensions while rejecting those of larger dimensions, these materials are known as "molecular sieves". These molecular sieves have been utilized in a variety of ways in order to take advantage of their properties.
Molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of SiO.sub.4 and Group IIIB element oxides, e.g. A10.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms. The ratio of the total Group IIIB element, e.g. aluminum, and Group IVB element, e.g. silicon, atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIB element, e.g. aluminum, is balanced by the inclusion of a cation in the crystal. Examples of such cations include alkali metal or alkaline earth metal cations. This can be expressed wherein the ratio of the Group IIIB element, e.g. aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, with ratios from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up, as measured within the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724; 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content.
Additionally, aluminum phosphates are taught in the U.S. Pat. Nos. 4,310,440 and 4,385,994, for example. These aluminum phosphate materials have essentially electroneutral lattices. U.S. Pat. No. 3,801,704 teaches an aluminum phosphate treated in a certain way to impart acidity.
An early reference to a hydrated aluminum phosphate which is crystalline until heated at about 110.degree. C., at which point it becomes amorphous or transforms, is the "H.sub.1 " phase or hydrate of aluminum phosphate of F.d'Yvoire, Memoir Presented to the Chemical Society, No. 392, "Study of Aluminum Phosphate and Trivalent Iron", Jul. 6, 1961 (received), pp. 1762-1776. This material, when crystalline, is identified by the JCPDS International Center for Diffraction Data card number 15-274. Once heated at about 110.degree. C., however, the d'Yvoire material becomes amorphous or transforms to the aluminophosphate form of tridymite.
Compositions comprising crystals having a framework topology after heating at 110.degree. C. or higher and exhibiting an X-ray diffraction pattern consistent with a material having pore windows formed by 18 tetrahedral members of about 12-13 .ANG. in diameter are taught in U.S. Pat. No. 4,880,611.
A naturally occurring, highly hydrated basic ferric oxyphosphate mineral, cacoxenite, is reported by Moore and Shen, Nature, Vol. 306, No. 5941, pp. 356-358 (1983) to have a framework structure containing very large channels with a calculated free pore diameter of 14.2 .ANG.. R. Szostak et al., Zeolites: Facts, Figures, Future, Elsevier Science Publishers B.V., 1989, present work showing cacoxenite as being very hydrophilic, i.e. adsorbing non-polar hydrocarbons, only with great difficulty. Their work also shows that thermal treatment of cacoxenite causes an overall decline in X-ray peak intensity.
Silicoaluminophosphates of various structures are taught in U.S. Pat. No. 4,440,871. Aluminosilicates containing phosphorous, i.e., silicoaluminophosphates of particular structures are taught in U.S. Pat. Nos. 3,355,246 (i.e. ZK-21) and 3,791,964 (i.e., ZK-22). Other teachings of silicoaluminophosphates and their synthesis include U.S. Pat. Nos. 4,673,559 (two-phase synthesis method); 4,623,527 (MCM-10); 4,639,358 (MCM-1); 4,647,442 (MCM-2); 4,664,897 (MCM-4); 4,638,357 (MCM-5); and 4,632,811 (MCM-3).
A method for synthesizing crystalline metalloaluminophosphates is shown in U.S. Pat. No. 4,713,227. An antimonophosphoaluminate and the method for its synthesis are taught in U.S. Pat. No. 4,619,818. U.S. Pat. No. 4,567,029 teaches metalloaluminophosphates, and titaniumaluminophosphate and the method for its synthesis are taught in U.S. Pat. No. 4,500,651.
The phosphorus-substituted zeolites of Canadian Patents 911,416; 911,417; and 911,418 are referred to as "aluminosilicophosphate" zeolites. Some of the phosphorus therein appears to be occluded, not structural.
U.S. Pat. No. 4,363,748 describes a combination of silica and aluminum-calcium-cerium phosphate as a low acid activity catalyst for oxidative dehydrogenation. Great Britain Patent 2,068,253 discloses a combination of silica and aluminum-calcium-tungsten phosphate as a low acid activity catalyst for oxidative dehydrogenation. U.S. Pat. No. 4,228,036 teaches an alumina-aluminum phosphate-silica matrix as an amorphous body to be mixed with zeolite for use as cracking catalyst. U.S. Pat. No. 3,213,035 teaches improving hardness of aluminosilicate catalysts by treatment with phosphoric acid. The catalysts are amorphous.
Other patents teaching aluminum phosphates include U.S. Pat. Nos. 4,365,095; 4,361,705; 4,222,896; 4,210,560; 4,179,358; 4,158,621; 4,071,471; 4,014,945; 3,904,550; and 3,697,550.
The precise crystalline microstructure of most zeolites manifests itself in a well-defined X-ray diffraction pattern that usually contains many sharp maxima and that serves to uniquely define the material. Similarly, the dimensions of pores in these materials are very regular, due to the precise repetition of the crystalline microstructure. All molecular sieves discovered to date have pore sizes in the microporous range, which is usually quoted as 2 to 20 .ANG., with the largest reported being about 12 .ANG..
Certain layered materials, which contain layers capable of being spaced apart with a swelling agent, may be pillared to provide materials having a large degree of porosity. Examples of such layered materials include clays. Such clays may be swollen with water, whereby the layers of the clay are spaced apart by water molecules. Other layered materials are not swellable with water, but may be swollen with certain organic swelling agents such as amines and quaternary ammonium compounds. Examples of such non-water swellable layered materials are described in U.S. Pat. No. 4,859,648 and include layered silicates, magadiite, kenyaite, trititanates and perovskites. Another example of a non-water swellable layered material, which can be swollen with certain organic swelling agents, is a vacancy-containing titanometallate material, as described in U.S. Pat. No. 4,831,006.
Once a layered material is swollen, the material may be pillared by interposing a thermally stable substance, such as silica, between the spaced apart layers. The aforementioned U.S. Pat. Nos. 4,831,006 and 4,859,648 describe methods for pillaring the non-water swellable layered materials described therein and are incorporated herein by reference for definition of pillaring and pillared materials.
Other patents teaching pillaring of layered materials and the pillared products include U.S. Pat. Nos. 4,216,188; 4,248,739; 4,176,090; and 4,367,163; and European Patent Application 205,711.
The X-ray diffraction patterns of pillared layered materials can vary considerably, depending on the degree that swelling and pillaring disrupt the otherwise usually well-ordered layered microstructure. The regularity of the microstructure in some pillared layered materials is so badly disrupted that only one peak in the low angle region on the X-ray diffraction pattern is observed, at a d-spacing corresponding to the interlayer repeat in the pillared material. Less disrupted materials may show several peaks in this region that are generally orders of this fundamental repeat. X-ray reflections from the crystalline structure of the layers are also sometimes observed. The pore size distribution in these pillared layered materials is narrower than those in amorphous and paracrystalline materials but broader than that in crystalline framework materials.
Indeed, X-ray diffraction patterns have come to play an important role in identification of various crystalline materials, especially pillared layered materials. Nevertheless, it is the physical properties of these materials which render them valuable assets to the scientific and industrial communities. These materials are not only valuable when employed in the petroleum industry, but they have also been found to exhibit properties useful for a variety of applications including such fields as nonlinear optics and the biological and chemical sciences.
One particular area of interest involves employing these porous crystalline materials in the fields of chemistry and biology in order to effect the separation of substances contained within a mixture. Generally, separations encompass a vast number of apparatus and techniques; however, they can be broadly classified into several categories, namely: extractions, distillations, centrifugations, precipitations, filtrations, clarifications, membrane separations and chromatography.
Despite the vast range in apparatus and techniques, all separations involve the need for two phases of matter. For example, extractions generally involve two immiscible liquid phases, typically an aqueous and an organic phase, wherein a solute migrates out of one phase and into the other based upon its relative solubility between the two phases. In contrast, chromatography generally involves a fluid mobile phase which is contacted with a fixed stationary phase.
In chromatography, the mobile phase is typically a gas or a liquid which contains a sample to be separated or purified. The mixture generally contains several components which are to be isolated from one another based upon some physical property of the substances, such as molecular weight, binding polarity or the like. Additionally, all chromatography methods operate to isolate a particular substance based upon a retention of that substance by the stationary phase or, alternatively, by the relative tendencies of different substances to partition into the stationary phase and become associated therein.
There are various types of chromatography, including ion exchange, reverse phase, partition, affinity, elution, column, adsorption, flat-bed, batch, thin layer, paper, gel permeation and other size exclusion-based chromatography as well as gas, liquid and solid chromatography. Generally, the nomenclature for the different types of chromatography is based upon either the type of mobile phase employed, the nature of the stationary phase, the nature of the interaction between the stationary phase and the substance retained by it or the type of technique or apparatus used in the chromatographic system. For example, gas and liquid chromatography are named for the type of mobile phase employed. Ion exchange, affinity, partition, adsorption and size exclusion chromatography are named due to the nature of the interaction between the stationary phase and the substance retained by it. Elution, reverse phase, column, flat-bed, batch, thin layer and paper chromatography are named based upon the type of technique or apparatus employed.
As previously mentioned, some other separations include filtration, clarification and membrane separations, all of which are important in the fields of chemistry and biology. Membrane separations are processes for the separation of mixtures using thin barriers or membranes positioned between two miscible fluids. Typically, concentration or pressure differentials provide a suitable driving force across the membrane for promoting separation of one or more components in the mixture.
Generally, membrane separations may be sub-divided into the categories of ultrafiltration, dialysis, electrodialysis, reverse osmosis, gas or liquid diffusions and facilitated transport mechanisms driven by chemical reactions. Filtration involves the separation of solid particles from a fluid-solid suspension in which they are contained. The separation is generally performed by a filter medium having a predetermined pore size. Clarification involves the removal of extremely fine, particulate solids from liquids.
Although clarification can be thought of as an ultra-fine filtration in the sense that solid particulates are removed from a liquid, the technique is different from filtration in that clarification employs a different set of separation mechanics including gravity sedimentation, centrifugal sedimentation, magnetic separation and similar mechanical separation techniques which usually do not involve a filter. Other major separation techniques include distillation, extraction and precipitations.
Regardless of the type of separation or the specific parameters associated with it, all separations can be grouped into two categories, those which employ solids either as a separation means or as a support mechanism for the separation means and those which employ no such solids. For example, extractions usually do not employ solids for the separation but rather rely upon the relative solubility of a particular solute in two immiscible liquid phases. In contrast, most chromatography-based separations employ a solid in some form, either as a support mechanism or a separation means or both. Alternatively, distillations and precipitations may employ solids but these are typically utilized either as inert condensation supports or crystal seeding mechanisms, respectively.
In liquid-liquid chromatography, for example, one liquid functions as the mobile phase while the other operates as the stationary phase. The liquid stationary phase is typically supported by affixing it to a solid substrate, either by physisorption or chemisorption. The separation occurs via the relative tendency of certain components present in the liquid mobile phase to partition into the liquid stationary phase based upon some physical characteristic, such as polarity or solubility. This form of chromatography is also commonly referred to as partition-based chromatography since it is based upon the tendency of a sample to partition into one phase more readily than other components in a mixture.
Alternatively, most affinity-based chromatography techniques employ a stationary phase which involves a solid separating agent supported by an inert solid support means. Typically, an inert solid having a high surface area is coated with a substance, such as a particular protein, which has a specific binding affinity for a substance which is to be isolated. In such a system the solid protein-based separating agent performs the separation.
Other types of chromatography systems employ solids which function as separating agents as well as support mechanisms. For example, certain types of adsorption-based chromatography systems employ porous solid materials to perform a size exclusion-based separation while simultaneously providing a support to which a secondary separating agent is attached. The secondary separating agent is typically some reactive molecule or a functional chemical moiety that performs a separation based upon its interaction with a particular component in a mixture.
Additionally, other separation techniques may employ solids in the manner discussed above. For example, membrane separations often employ solid porous materials which have been deposited or coated onto an inorganic substrate in order to produce a thin layer membrane employed as a separating agent. Similar approaches are used for constructing filters used in filtration and clarification procedures.
In light of the various separations which employ solids, such as those discussed above, there exists an ongoing need to develop new and useful separation techniques as well as the need to provide vehicles for improving the efficiency of existing separation technologies. As previously mentioned, a variety of porous inorganic solids, such as zeolites as well as other related types of molecular sieves, have been applied in wide range of technologies due to their unique physical properties. In particular, these materials have been employed in separation techniques, mostly in the area of industrial gas separations, since the porous nature of their microstructure allows entering molecules access to an extensive surface area. This increased surface area enhances the utility of these materials with respect to their catalytic and sorptive activities.
Unfortunately, many of these materials have some inherent variation in pore size which tends to undermine the integrity of separations in which they are employed, especially when these materials are involved in size exclusion-based separation techniques. Moreover, most currently available molecular sieves have a relatively small pore size. As there is no way of tailoring the pore sizes of such sieves, the artisan is forced to choose between different sieves, depending upon the desired pore size. This is often undesirable since utilizing different sieves may invariably invoke unpredictable results in the procedure in which the sieve is employed.
Additionally, many currently available sieves do not readily accept the attachment of functional groups to their pore walls, thereby limiting the range of separation applications in which they may be employed. Furthermore, many of the large pore molecular sieve materials which are currently available lack thermal stability and often behave unpredictably in response to changes in pH.
It is therefore an object of the present invention to provide a new method for separating substances using a unique, large pore crystalline phase material which is thermally stable under a range of pH while exhibiting a high degree of uniformity in pore size.
It is further an object of the present invention to provide a new method for separating substances using a unique, large pore crystalline phase material which has pore walls that can be functionalized to include various reactive chemical moieties, as well as affording a high degree of control for the artisan attempting to modify the material in order to provide a pre-determined pore size.